Electrical network system



6 Sheets-Slxeet-l July 26, 1938. N. WIENER ET Al.

ELECTRICAL NETWORK SYSTEM Filed July 18, 195e Il lll July 26, 1938. N. WIENER ET A1. 2,124,599

V ELECTRICAL NETWORK SYSTEM /V VA/7.0195 mi Win/m.

www Ll July 26, 1938. N. WIENER ET AL I 2,124,599

ELECTRICAL NETWORK SYSTEM Filed July 18, `1936 6 Shets-Sheet 3 .00a- /1 0 (zo /J'a\/ /80 Marsi/117? m Degree: n .w-- 7002 u .004 G1004 gx E faQ/2. lg H006 i* y .A /NVf/Was A faQ/6 W WW ww im July 26, 1938. N. WIENER x-:T Al.

IEILIICTRIGAL NETWORK SYSTEM Filed July 18, 193e e sheets-sheet 4 July 26, 193s. N. WIENER ET AL 2,124,599

ELETRICAL NETWORK SYSTEM I Filed July 1s, 195e e sheets-sheet 5 960% Z40db The number: wv Me na/ /fe'rs refer fa #ze laas; bonds in F924.

July 26, 1938. N. WIENER ET Al.

ELECTRICAL NETWORK SYSTEM Filed July 18, 195e e shee-sheet e 5b QD Bh.

Patented July 26, 1938 UNITED STATES ELECTRICAL NETWORK SYSTEM Norbert Wiener and Yak-Wing Lee, Peiping,

China, assignors to American Telephone and' Telegraph Company, a corporation of New York Application July 18, 1936, Serial No. 91,360`

15 Ciaims. (o1. 17e- 44) This invention relates to wave transmission networks and; more particularly to networks which pass one or more regions of the frequency spectrum with substantially uniform attenuation 6 while subjecting other regions to very great relative attenuations. It has as its principal object the improvement in the art of constructing suchV networks as will divide the frequency spectrum into a large number of regions and will furnish a large number of output circuits in each of which one of these regions will be transmitted with small and substantially constant attenuation while other regions will be greatly attenuated. It has as a further object the improvement of 15` the art of separating each of a plurality of input electromotive forces or currents from its side bands, and of combining the electromotive forces or currents thus separated into a single current which may be transmitted ior communication purposes over a single circuit. It has as a further object the attainment of these ends by a structure which contains only a small number oi kinds and sizes of element repeated a great number of times, and thus lends itselfv to methods 2 of mass manufacture. It has as a further object the construction of a filter with a plurality of.

outputs or oi inputs, and of excellent linearity vof phase shift, which is made possible together with an economy oi parts by the fact that the phase shift of each output or from each input is determined by a network structure common toall.

These objects are achieved in accordance with the invention by constructing the network in the form of a cascade oi networks, each with a multiplicity of outputs, such that the rst member ofthe cascade separates the frequency spectrum` into a number of regions, several of which are transmitted with low attenuation in each output of the first member, while the later members of the cascade complete the isolation of these regions. They -are further achieved by the construction of the component networks oi the cascade or of certain of them as themselves cascades of identical phase shifting networks, together with means of combining outputs taken from between the stages of such cascades into sums having desired attenuation with respect to the original input. In case the device is used for the suppression of side bands, the roles of the inputs and the outputs are interchan-ged, andthe cascade structure is reversed in order. The structure remains the same, except that all amplifiers are reversed in sense. Throughout this specification, this reversal is to be understood-as ineluded in our description, wherever it is approprate.

In the component cascades of phase shifting networks, a considerable economy is obtained by 60 the use of an open circuit termination to produce a reflection, and thus to substantially double the length of the cascade. While only open circuit reflection will be discussed in the examples fully described in the present specification, '1e inventors are fully aware of the possibility of producing analogous results by short circuit reflection and other modes of reflection, and have included these other forms of reflection in their claims.

Other objects and structural details of this invention will be apparent from the following descriptio'n, when read in connection with the aecompanying gures, wher-ein:

Figures 1, 2 and 3 are illustrative oi network elements used in the invention;

Figure 4 shows a network configuration basic to the invention;

Figures 5, 6 and '7 represent different embodiments of the invention;

Figures 8 and 10 show two embodiments vof a modied form of the invention;

Figure 9 is illustrative of the characteristics ofthe systems of Figures 8 and 10;

YFigures 11, 12 and 13 representtypical selectivity characteristics obtainable by means `of the invention;

Figures 14 and 15y illustrate the use of repeaters in the systems o-f the invention;

Figure 16 shows a second modified form of the invention;

Figures 17 and 18 are illustrative of the transmission characteristics of a system in accordance with the invention;-

Figures 19, 20, 21 and 22 are illustrative of particular phase shifting networks used in the systems of the invention;

Figure 27 shows the use of coaxial transmission lines as phase shifting devices in the systems of the invention. Figure 1 represents a lattice structure comequal impedances Z connected diagonally between the input and the output Yterminals, We

suppose both Z and Z to bepure imaginariesthatA is, the impedances are reactive. suppose that the pure resistance R is a mean proportional between Z and Z'. This structure We also prisingy two equal line impedances Z and two.4k v

represents one formthat certain important ele-c ments of the system of the invention may take. The Vresistance R. representsl the terminal load of the lattice, and also the load out of which the lattice works. The impedance of the lattice plus'` terminal load vwill also be R. The absolute voltage ratio of the lattice viewed as a four terminal Figure 2, all with absolute voltage ratio unity,

the 'terminals of the Vgenerator'is u, the voltage network will be unity, and the angle of phasel shift generated by the lattice will beY Y' il Y ftzin-lj. (i).V

' Figure 2 represents'a cascade of lattices suchn fas those of Figure 1. /We shall again assume that the branch inipedances are all reactive, and that the mean proportional of the line impedances Vand the corresponding lattice impedanc'es-is the same vconstant resistance R.Y foreach section. The structure of Figure 2 will vagain have abso-Y lute voltage ratio unity when .terminated by a yresistance R, While its angle of 'phasej'shift will be 0- n 2 t Pr@ `(2) an J R In the above equationv and in those that follow, n denotes the number ofY junction points or terminal pairs in the sectionalized line, the num- V`bier of sections. being V11i-1, and 7c denotes the to the actual generator of the Vimage generator, and willV be v may be :determined numerical orderin the sequence ofY any given line section' or itsinput terminals.

Figure 3 represents acascade of identical four terminal f networkssuch as those of Figure 1 or image resistance R, and 'phase shift angle 0, where @is a function of Ythe frequency j. If the complex voltage across terminals l is 1J, the'complex Y voltage acrossV terminals 2 will be De, and across terminals 'u will be UMa-1W. Y

Figure 4 represents a cascade such as'that of Figure 3, but'diff-eringvfrom that oi'Figure '3 in that while the cascade leads out of a resistanceV R, itleads intoan open circuit.i The dottedlines indicate the reflection ofthe structure of .Figure` 4in the point where the circuit open, and Vtogetherwith the' full Ylines indicate an'equivalent structureoi` twice the numb-er of meshes and fter-` minated lov-aY resistance.V If the voltage across acrosstermin'als lwill be theV sum of that due lligure and the u guinea-zia); (3) The voltage across terminals 2 will be,

' iainV-sm), v Y (4) andthe voltage across the remaining `terminals similarly, that across terminalsu being f I Letv us represent this voltage by u1. i It differsl only by a phase' shift lofl (uf-1)@ from u. The Voltage across terminals lc, in ouresystem of nu-Y mration, will be w cos (n-IH.

I, Thus if we provide means: `(a) of multiplying -the"voltag'esv (6) vby real factors, which may be #seing this end by the use of vacuumtubes with positive 'or negative; and1(b) of adding these products; weshall be able to produce a'fou'1-Y terminal network With a anda voltage ratio of y There are variouspracticablemethods ,pro-

ducing Vthe Vnecessary multiplications, and phaseY shiftsvby Ahigh resistance. leads, Vacuumtubes.' etc. Figure 5, represents onelmethod of 4achievadjustable gain. In this structure mik is the voltage ratio of the Vlcth amplier from the end more remote from the input, the voltage ratio of the terminal amplier being uan. A reversal of the output terminals or the input terminals .of .the amplifier is considered equivalent to a change in sign in the voltage ratio.

is connected to the middlepoint of the .potentivorneters, while the other output terminal is connected by leads of extremely high and equal .repotentiome- Figure 6 indicates a .method of achieving the same result by the use of high resistance leads inversely proportional'iny resistance' to 'the Vconstants ak. Figure 7 indicates yet another method in which high resistance potentiometers are used to determine the aks, an'djin which one output terminal sistance to the points on the'several ters determined by the aks.

Y It willbe seen that all these methods of connectingthe output terminals to the main structure of a cascade ofY phase shifting networks do not affect to any great ldegree the characteristics of this cascade, as they lead actually or approxi-` mately into open circuit. Thereis thus no dili'if culty iniprinciple in connecting the same main structure to as many output circuitsas may be desired, each output circuit having an independent attenuation characteristic with respect to A the input, although they share the same phase characteristic, apart from possible abrupt changes of 180 in phase where the attenuation characteristic is infinite. This use of multiple' outputs is illustrated in Figure 8. In this figure ampliers V1 to 12, inclusive, are connected at their input terminals in groups of three to the junction points of the line sections and' at their out? put terminals in groupsof four to the ultimate terminals Ydesignated outputV I, output V2, and Voutput 3. The arrangementis such that .each

of .the three outputs receives oscillations from each1of thefour junction' points in the line appropriately modied by the Arespective amplier gains. The disadvantage of such an arrangement is that if is carried out with Vthe aid of amplifiers, the numberof amplifiers is likely to be somewhat excessive, while if high resistance s leads are used, the parallel connection of a large number tends to decrease their resistance unduly and to make it possible to treat this resistance as infinite to a rst approximation. It is desirable in using such a network with multiple outputs to restrict the number of outputs as far as possible. Figure 8 is to be read in connection with the following tables.

Voltage ratio Voltage Voltage Voltage Voltage Voltage Voltage Voltage ratio of output 1 f (au-l-aycos 0|a2 cos 20|a3 cos 30) `Voltage natio of output 2 I *5020+131 cos 0-172 cos 20-i-b'3 cos 30) Voltage ratio of output 3 'i-CD+G; cos @+02 cos 26-l-c3 cos 30) One of the main uses for a network with a multiplicity of outputs is as a bank of wave filters, to split a given range of frequencies into several approximately equal ranges, and to deliver to each output circuit one and substantially only one of these ranges with substantially uniform small attenuation. This may be done in two or more stages, the first stage being the separation of the frequency band as by a pair of complementary multiple band pass iilters into two parts as illustrated in Figure 9, each part consisting in a succession of approximately equal bands separated by approximately equal intervals. The second stage and the later stages of filtering (if any) will be devoted to the suppression in veach output of all but one of the bands passed. This suppression does not demand an attenuation characteristic which changes rapidly in a narrow interval of frequency, and may thus be attained by a secondary filter of simple and inexpensive construction. Accordingly, the original stage of filtering will only have two separate outputs, thus greatly economizing in the use of amplifiers. A further economy results from the fact that the phase shifting networks between outputs are designed to produce approxmately linear shifts of a large number of complete revolutions, so that substantially the same attenuation pattern shall be repeated many times in the frequency scale. This means that the leads to amplifiers are located at infrequent intervals along the lattice structure, which again reduces the number of amplifiers.

If only two outputs are desired, and these complementary, the Fourier series for the two will have respective forms ao--al cos 0-l-a2 cos 20+ -l-a cos n0 (8) and a0-a1 cos 0+a2 cos 29- ian cos n0 (9) This suggests that the odd and the even amplifiers may be connected by resistance leads which will respectively add and subtract their outputs. This connection is illustrated in Figure 10. It is of course understood that the impedance across output terminals l is the same as that across output terminals 2. We shall assume the resistance looking backward into either of these pairs of terminals to be equal to the image resistance of our unit phase shifting network. This becomes important when the outputs are connected to the inputs of subsequent similar filtering devices as described later in connection with Figure 23. l

The problem of designing a network such as that of Figure 10, as the first stage of a multiple output lter, may be divided into two parts. In the first, we express the characteristic of the outputs irr terms of 0, the phase shift or phase constant of the networks lying between amplifier connections, and in the second, we discuss these phase shifting networks, and 0 as a function of the frequency f. In the firststage We shall assume that 0 is to be roughly proportional to f, which is one of the results we secure in the second stage in design. The mathematical problem of the rst stage is accordingly: to find a polynomial (8) which shall be nearly equal to 1 over Oi, where 01 90, and which shall be small over 6180". Over the intermediate region 61 0 90, the behavior of (8) is not held to any veryV strict requirements, except that it is desirable that it shall not exceed 1 in modulus. It will be seen that there are several quantities which should simultaneously be kept small.` These are: (a) n, the order of the polynomial (8); (bi) 90-01, the width of the transition band; (c) the upper bound of the modulus of (8) over (90, 180); (d) the upper bound of the difference between (8) and 1 over (0, 0i). The relative Weight to be given to these several quantities is impossible to determine in advance, and accordingly we shall not attempt any purely mathematical minimization of these four quantities, but shall be content with a method of determining n and the aks which has worked very Well in practice. It has the advantage over several other methods which we have tried that the computation is not unduly tedious. ItV depends on giving to the polynomial (8) assigned values 1 at equally spaced points in the desired pass band and values 0 at equally spaced points in the attenuation band, and on choosing the values in an intermediate transition band in such a manner as to make (8) as smooth as possible in points remote from the transition band. It is thus va method of trigonometrical interpolation. The results of this interpolationare quite good as they stand but a little experimental manipulation may subsequently be used to reduce some of the remaining irregularities.

In the process of interpolation the following trigonometrical identities are made use of. These identities may be proved by standard mathematical processes.

The function which results from trigonometrical interpolation between the value 1 at 0:0 and the value 0 at 0=k1r/n (0 7cn) is Where 0(0) does not exceed in absolute value a constant multiple of 9.

The function sin (t9-) n ity equals au. We wish to determine the sequence of values of av whichl will minimize the order of It thus suggests itself that' (afg. l'

It not dimmi to establish this by matlassianv cal induction. Thus theV partial fraction expansion (13) Vis as small in order as possible for great values of 0, andit becomes reasonableto put large in one region and relatively as snall as possible away from that'region. As we`have seen,

this function may be fvery accurately r'epresentefd by a trigonometrical'polynomial. Y l v If We take Vsuch a polynomial,- and replace by we merely displace Vthe peak.V f Ii"v wey addfa large number of such polynomials in Whichthe have beenrsuccessively displaced by peaks we obtainY a function which Ynearly a constant over one region, nearly zero over another, and

which A'assumes intermediate values .over two transition bands. It may be obtainedby trigonometrical interpolation between' values wh-ich are 0 overV equally spaced points inthe attenuation region, and then proceeds through the successive values Y Y Y (wma-Www, Y., 1, v

thereafter remaining l until the next transitionl Y Aregion is reached, where the same values. are as- V sequences of values run:

sumed in the opposite sequence.

sla

poooovo `nniial'fc'if'given degree which'f'is to'fbe very-close Such transition' to 1 over one range and Very close to 0 over'another, we may use trigonometric interpolation with these transitional values. As an example,v let us interpolate between the values Angie 0 15 30 45 00'75 90 105 120 135 165 180 *(17) Function 1 1 1 1 .75.250 0 0 0 0 0 Here60 and V'7'5" Yare transitional values. We

' obtain-as our interpolation function y,

.S75-P5798 cos 6+.21242 cos 20-.07113 cos y30 .125" cos kL10-.03297 Ycos 50-1-04167 cos `60 -l-.03596 cos 707.0122c0s 90-.00409 `(30s 10H +.00055 cos 110. Y (18) The graphof this is shown in Figure `11. Anr enlarged view of the portion of this graph between 90 and 180 is shown in Figure 12. It Will be seen that thisgraph has one rather large peak after 90. By a little experimenting, this may be removed, and if we replaceV (18) by .37595+.57980 cos 524-1217032 cos 20-.0'7113 cos -.12290 cos 40-.03297 cos 50+.03957 cos vEiffel-.03596 cos 7a4-.00210 cosY 80-.01220 cosY 90-.00514 cos 100, (19) vFigure 12is'rep1aced by Figure 13.

The second stage of the design of our invention lconsists'in the design of phase shifting networks which shall give a substantially linear phase shift overY a wide band of frequencies. There arermany structures available for this purpose in the art, and We make no claim for originality'in'this respect. U. S. Patent No. 1,828,454

issued to Bode Oct. 20, 1931 discloses Such a structure on page `8, and U. S.*Patent No; 1,792,523 issued to Zobel Feb. 17, 1931 give examples of such structures. Figure 19 illustrates a form of phase Y shifting network used in oneA realization of our invention. The magnitudes ofthe Various inductances and capacities are indicated in the Vchart accompanying the diagram. Y

In general, in any electricalY network-design, Vthere are two stages:V (1) the design made under the assumption that the elements are pure resistan'ces, inductances, and capacities, with values precisely assignable at will; and (2) the modifications made in orderV to take account of parasitic resistances and leakages and of other imperfections of thel elements, together with the changes indesign conditioned by: the fact that very close tolerances are economically impracticable. The methods used inV overcoming these diiculties are much the same for all classes of network. A pure phase` shifting network which is modied by giving'tofiea'ch inductance a parasitic resistance. in series andY to each capacitance a parasitic leakage inparallel, and in which the ratio of each inductance to its accompanying resistanceis the same as the ratio of each capacitance to its accompanying leakanceghas the same phase shifting characteristic as` the corresponding network ofipure reactances, and a xedbut non-zero vattenuation. -For this, see Bode, U. S. Patent 115828,454' issued Oct. 20, 1931, p. 8,.1ir1es 'T8-491.V

The' attenuation Vof such a network may be cancelledV by Vthe use Yof repeater elements of 10W gain. One arrangement rof such elements is shown in` Figure 14. It is not thereby intended to restrict our invention to any particular arrangement of'suchielements. A terminal repeater'element maybe usedto eliminate the losses in the final phase'shifting element upon reflection; One

ofthe many possible connections Yof said repeater lementisshowninl'igure 15.. f

VItC is" Wellknown in the art that a practicable method of avoiding the need for! excessively close tolerances in the construction of a network is to realize this as a cascade of subordinate networks. This method is at our disposal here as well. Figure l represents one embodiment of our invention in a vsingle structure, and Figure 16 represents an equivalent embodiment of our invention as a cascade of simple structures. While the circuit shown in Figure 16 includes more elementary line sections than that shown in Figure 10, and, therefore, requires the use of a larger number of capacities and inductances, the permissible manufacturing tolerances in the elements is greater and the increasednumber is offset by lower individual costs. The setting and structure of Figure A16 are obtained as follows: (1) the voltage ratio is plotted against 0; (2) thereal zeros ai, am of (20) are determined; (3) the polynominal is expanded; (4) the polynominal P(0) /Q(0) is determined at a number of points (generally equally spaced) equal to the number of coeicients in its Fourier expansion; (5) `the polynominal P09) /Q(9) is then evaluated by interpolation; (6) Q(0) is then divided into two factors Qi(0) and Q2(0) in such a way that Q1(0) and P(0) /Q1(0) shall be roughly of the same order of magnitude in the attenuation band; (7) a network constructed to give two outputs P(0) /Q1(0) and its complementary output is then followed in cascade by two networks with characteristicsv Q1(0), one for each output.

The further compensation of our network for variability of resistances, capacitances, and inductances with frequency, depends on the particular materials at our disposal for construction, and falls outside the content of this invention.

The two outputs of our first stage of filtering themselves require subsequent ltering to separate their several pass bands. This may be done directly by means of filters of conventional design, or these outputs may be filtered by transmission through one or more networks of construction such as we have described in this invention, before the final separation-if any-is performed by lters of more conventional type. The secondary networks are so designed as to separate alternate bands passed by each output of the first filter. If the arrangement of these bands is as shown in Figure 17, the phase shifting networks of the secondary filter should shift the phase through the angle of those of the main filter. If the arrangement of the pass bands is as shown in Figure 18, the phase shifting networks of the secondary lter should shift the phase through this half angle, plus 180, the 180 shift to take place between frequency O and the lowest frequency passed. This may be done by combining phase shifting networks exactly similar to those used in a secondary filter of the other type in cascade with a single lattice element, this element consisting of two relatively large inductances in line combined with two relatively large capacities in the diagonal members of the lattice. Figure 19 represents one realization of the elementary phase shifting network, both of the main and of the secondary structure. This we term network A. Network B is one stage of the complete phase shifting structure of the main network, and networks C and D are respectively single stages of the phase shiftamiga/99` The design of the characteristics of the second' ary filters resembles that of the primary lters, but issimpler in View of the much less stringent requirements, and may be carried out empirically. The use of repeaters to cancel attenuation is permissible in the secondary filters as well as in theV primary filter, although the need for these will be somewhat less in View of the smaller length of the cascade in these filters. In this connection, it should be remarked that in our drawings, where a structure is indicated by repeater, its gain is to be just enough to compensate for pairasitic attenuation in the adjoining phase shifting sections. f

The tertiary or final lters need only have very low coverage, as .the intervals between the frequency bands passed are wide comparedY with the widths of these bands. They may be of the familiar Campbell type.

A complete assembly of one realization of our invention is diagrammatically indicated in Figure 23. The relative voltage amplications of the several amplifiers are indicated. The output attenuations of this lter are given as'functions of the frequency in Figure 24.

They system shown in Figure 23 comprises a pluralityof cascade connecte-d filters each of similar construction to that shown in Figure 10, and each having two pairs of output terminals to which succeeding filters are attached. The system is designed for the separation of twenty transmission bands spaced at even intervals of 3600 cycles, and for insertion between circuits having impedances of 600 ohms. The first lter, F1, comprises a phase shifting line, 2|, made up of seven similar tandem connected sections, labeled B in the drawings, and each being constituted by a plurality of the networks shown in' VFigure 19 connected vin cascade as in Figure 20.

source-G of 600 ohms impedance, thismatching" the characteristic impedanc of the line. From the ends 'of the line and from the intermediate junction points, circuits branch to the inputs of 'eight amplifiers having preassigned gains as indicated in the drawings. The outputs of one ground of alternate amplifiers are connected in multiple to a common line, designated line I, and the outputs of the other group are connected to a second common line designated line 2.

Connections are made from these common output lines, to two pairs of output terminals, T2, T2' and T3, T3', through a system of eight equal resistances R arranged as shown in Figure 10. By this connection, the component voltages derived from line 2l are impressed in like phase on terminals T2, T2' and with phases reversed alternately on terminals T3, T3'. As already explained in connection with Figure 9, this results in the realization of different sets of transmission bands at each pair of terminals, the bands of the one set alternating in the frequency scale with the band of the other set and being substantially separated from each otherv in each output.

To terminals T2, T2" is connecteda second filter F2 which is generally similar to that shown in Figure 16. This lter comprises a phase shifting iine 22 having'.tiievsctionsnabeied B, similarl to theindividual sections of line 2 I. Y Four amplifiers having. gains Vrrespectively as noted in the a drawings arer connected to-theline terminalsand junction :points and are multiplied to a common line, designatedv line 3, at their outputs. At this pointthe filterV F2 differs somewhat from that shown inFigure 16. In thatrfigure, as in -lter FL Y the oscillation voltages derived from the phaseshifting lineare impressedLalternately on two commonV lines and .then transmitted through al system. of eight resistances to twopairs of terminals atwhich the alternate bandsappear. In thelter- F2Y the. derived oscillations are. transv mitted' toy the; pairs of terminals T4, T4' and T5,.T5-.from the singlecommon output 'of the amplifiers by'two paths one of which includes Va -phaseshiftingnetwork C of the type shown in Fig. 21. f This circuitarrangement4 and the use of the `networkV C produces. the equivalent of Vthe phase reversalslobtained 'by the output connections inthefilter F1 and-atthefsametime produces the phaseshif-t made,V necessary by the particulareband. allocations. resulting. from the first selection process. The. final output'terminals of filter F2 are Te, Te" and Tfn Tf1 Ywhich. are-connected to terminals Ti; T4 and T5, -Tsv respec- Y tively bynetworks N4 and N5 corresponding to the similarly disposed networks in Figurel. Ther phaseshifting networks, labeled C, in one branch of eachVv of V`these networks are alsoV of thetype vshown. in Figure'21.AV s Y The separation Veffected by filters F1 and F2 resultsin alwide separation ofthe bands appearing at terminalsT, Tsand Tv, TFH-s0 that the final `separation canfbe effected by 'simple filtersvr ofthe Ywell-known Campbell type.- In the particularrexample illustrated thebahds appearing at terminals T6,T6', inV the l,orderof their mid-v frequencies,- are numbers 4, 8, I2, I6 and 20, and

atterniinals Tf1, T7 the'bands are numbers 2, 6, Il) 4 and I8.. .The spacing ofthe bands is 14.4

kilocycles s o that only very. simple-filters are re-V e.. quired. for the nal separation.- In the drawings the Vfinal-,filters are designated bythe even numbers-2 to- 20L-which also'indicate the number ofA the corresponding'transmission'.band.. Y 1

1. For VVVthe final separation of theoddnumbered bands a second-filter yFais connected to the outputfterminals T3, T3 of-.flterrFi iat' which these bands appear. This-filter `and the y succeeding final filters 1aresimilar to-f'llter F2. and its associated. final. filters; butin lter F3 the secondary phasel shiftingu networks, labeled VDyeare of the type vshown inFigure 22 instead of the typeY C shown in. Figure 21.Y Thiscompensates for theY specific difference in the frequency allocation o1' the oddy numbered.` bandsrfrom that ofthe even numbered bands.

YThe phase characteristics of our outputs lneed a certain. consideration.v stage of ltering, by separate filters, the phase Y shift generated. by the Vfilter is the sum yof the i total. phase shift due vtothe'mesh'esof themain Y lter. inV cascade,- plus the phase shift .due-,totheY meshes of secondary filters in'cascade.V VA non-` Y linearity in these phaseV shifts willv have two .un-1

desirableeffects: (lf)Y it' will alter the relative widthfofthe different pass bands; (2) it will pro-A duce aV difference/of delay'A in transmission fromv i ther-effect. of producing affdiiferent delay in dif-v Y ferent pass. bands is`v usually innocuous. Y Effect' (Drissnearly-proportionalto rthe rst derivative of the phase shift with respect to thcrequency,

Except for Vthe final4 notveryrcritical, while effect (2) is nearly proportional ,to thegsecond derivative. rIn the exampleof Figure 23, the absolute delay of primary and secondary filtersl isf given as a function ofthe frequencyby the following table:

. f 0 n Y i (Kilocycles K per second) Delay (milliseconds) 3.20 2.9l 2.67 Y 2.91 3. 32 3.26 3.17 3.00

Fi'g`ures'25 and 26 represent graphically the data here lgiveirnumerically as to phase shift and delay. If Vthei'pa-ssbands dofnot eXceedrBOO cycles in width, 'the delay will accordingly not Vary over any band by more'than about .14 millisecond. The Width of the-different Vpass bands will not vary by a factor Yof morethanlg25. The delay characteristic of this filter, crude as the fundamental 'phasefs'hiftirig network is'acco'rd` Y ingly quite good. Y

The reasons for the relatively good phase characteristics of'our type of filter are:V (l) the fact that the phase shift Vofa single pass band differslfrolm one marginto'thelother by'only 180",Y

is' "noti thecase inY most formsv of Ywave filterV with 'oirily'faA single'output; (2)V as farA as the pha'se'cli'aracteristic is concerned, the extreme edgesiof thepass bandY do not' form a transition band difficult'of adjustment, as in other types ofwav'e filter, but'are quite'as'e'asy to keep linear in phase characteristic as the interior portions. Ano'therwayof stating muchY the same thing is 1o so so v4o 5oA 60 flo' ,th'follo'wing-the' pooling of the main structure Y f .largenumber.r of'distinct filters into a single filter makes it possible for each output to take'V advantage of the excellent phase characteristic of 4"asiiigl'e 'lt'er containing the totality of parts cfall thecom'ponentfilters. il Y, v

VThe.great'advantage of this type of filter over all known'typesisthe fact that the main structure-Handthe secondary structure V,consist in a repetition'- of a llarge' number of similar parts,

which only depend onY the general frequencyV range ito'b'e covered'b'y the'filter; while the particular." characteristics of V"the filter: are deter@ asthey equi-valent of a bank Vof filters together witl'li their appurtenant amplifiers.

"vWliile we have describedv in some detail one embodiment of our invention, its kfeatures may be colczibi'ned ina large number of different ways.'V Itisvv to be understood that the invention is'notv Y limitedto the exact` details of the'stru'ctures ills` trat'edvv and described, as numerous modifications thereof may beY made byrpersons skilled in the artwithout departing from Vthe spirit and scope of theinvention Y YAmong these besides Vmany othersare: the.V replacement of the device of the use -of open circuit refiection in a cascade of phase shifting .networks/by short circuitrreecvtiorior otherfforms of reilection-the substitu- .tionfof lengthsof coaxial line or other forms of i l-ine with"substantially constant attenuation over a--band of frequencies for the phase shifting networks already illustrate-d (-Va formy of network in# vcivingthischange isillustrated in Figure 27);

the replacement of electrical phase shifting netamigos Works by mechanical acoustical or supersonic structures. The types of phase shifting network used and the details of the connection of the several stages of our invention are also subject to large variations Without departing from the spirit of the invention.

What is claimed ist l. An electric wave selecting system comprising a wave transmission line terminated at one end to produce full wave reflection and at the other end to provide zero reflection, said line comprising a plurality of equal sections connected in cascade, a wave source connected to one end of said line, a common output circuit, a plurality of electrical paths connecting said common output circuit and the terminals of said line sections, and amplitude controlling means in each of said paths.

2. An electric wave selecting system comprising a wave transmission line terminated at one end to produce full wave reflection and at the other end in an impedance matching the characteristic impedance of the line, said line comprising a plurality of cascade connected sections having equal phase constants, a'wave source connected to one end of said line, a common output circuit, a plurality of separate electrical paths extending between said common output circuit and the terminals of said line sections, and amplitude controlling means in each of said paths.

3. An electric wave selecting system comprising a wave transmission line open-circuited at one end, a wave source of impedance equal to the characteristic impedance of said line connected to the other end thereof, said line comprising a plurality of cascade connected sections having equal phase constants, a common output circuit, a plurality of separate electrical paths interconnecting said output circuit and the junction points of said line sections, said paths presenting high impedances to said line at their junctions therewith, and amplitude adjusting means in each of said paths.

4. A selecting system in accordance with claim 3 in which each of the said sections of said transmission line is an electrical network of lumped inductances and capacities substantially free from resistance, said networks having constant resistance characteristic impedances.

5. A selective system in accordance with claim 3 in which the said amplitude adjusting means comprise space discharge amplifiers.

6. An electric wave selecting network comprising a substantially dissipationless wave transmission line terminated at one end to produce full Wave reflection and at the other end to suppress wave reflection, said line comprising a plurality of equal sections connected in cascade, a wave source connected to one end of said line, a pair of output circuits, a plurality of electrical paths connected respectively to said line at the ends thereof and at the junction points of said line sections, amplitude controlling means in each of said paths, means for impressing the outputs of said paths in like phase upon one of said output circuits, and means for impressing the outputs of said paths alternately in reversed phase upon the other of said output circuits.

7. The method of electric wave transmission which comprises reflecting an impressed wave, combining thev reflected Wave and the impressed wave, deriving from the combined wave a plurality of separate waves of like phase and different amplitudes, and combining the separate derived waves.

8. In a selective transmission system comprising a substantially dissipationless transmission line constituted by a plurality of similar cascade connected sections having equal frequency dependent phase constants, means for producing a transmitted wave and a s'mgle reflected wave in said line, means for deriving from the combined transmitted and reflected Waves, a plurality of separate oscillations of like phase and having frequency dependent amplitudes proportional to the cosines of and its integral multiples, 0 denoting the phase constant of said line sections, a common output circuit, separate circuits for impressing said derived oscillations onsaid output circuit, and means in each of said separate circuits for furthermodifying the oscillation amplitudesvin fixed ratios independent of the frequency.

9. A selective transmission system in accordance with claim 8 in which the fixed ratios of amplitude modification in said separate circuits are proportioned to provide substantially zero attenuation between the input of said transmission line and said output circuit for values of 0 in an assigned range betweenV Zero and 1r and to make the attenuation large for other values of 0.

10. In a selective system comprising a substantially dissipationless wave transmission line constituted by a plurality of similar cascade connected sections having equal frequency dependent phase constants, the method of selective transmission which comprises producing in said line a transmitted wave and a single reflected Wave, deriving from the combined wavesoscillations of like phase and having frequency dependent amplitudes proportional to the cosines of 0 and its integral multiples, 0 denoting the phase constant of the said line sections, modifying the amplitudes of said derived waves separately in assigned fixed ratios independent of frequency, and combining the modified derived oscillations.

11. A multiple pass-band Wave filter comprising a plurality of similar substantially dissipationless line sections connected in cascade, said sections having equal phase constants Which are substantially linear functions of frequency over a wide frequency range, means for impressing a complex wave on said line sections and means producing a single reflected Wave corresponding to each component of the complex Wave, means for deriving from the combined impressed and reflected waves a plurality of separatev oscillations of like phase and having amplitudes proportional to the cosines of 0 and its successive integral multiples, 0 denoting the phase constant of said line sections, a common output circuit, separate circuits for impressing said derived oscillations in like phase on said output circuit, and means in each of said separate circuits for further modifying the oscillation amplitudes independent of frequency in accordance with preassigned xed ratios whereby repeated transmission bands are provided at frequency intervals corresponding to the period of the variation l2. A multiple pass-band filter in accordance with claim 11, in combination with a second output phase, and means for impressingthe said derived oscillations upon said output circuit alterandtwo output Vcircuits, thetransrnission between Y said input circuit and 'one fof said output circuits 'beingcharacterized by a plurality of transmission bands separated by attenuation bands and the transmissionbetween the'input circuit and lthe other Y ofA said output Vcircuits Ybeing characterized by a ,plurality Aof transmission bands at frequencies within the attenuation -bands of the Vrstmentioned path, and additional multiple pass-VV band lters connected 'respectively to receive `waves from veachof said output circuits, each of said additional lters having two output circuits Vin which the successive frequency bands imwhich comprises separating and selectively transmitting the Waves in alternate frequency bands as Ya group and the waves in the other frequency bands-as a second group, and repeating the separation ofthe Yalternate'frequency bands in each group until the number of bands in each group isV reduced to unity. Y

l15. An electric Wave selecting system comprisi-Y ing in cascade a ilrst component lte'r selecting several sharply defined frequency bands and aV second component filter for selecting one of said bands from the output oi said irst component filter. Y

NORBERT WIENER. YUK-WING LEE. 

